Darth Vader's menacing castle first appeared in Rogue One: A Star Wars Story but its origin was not explained until the last few issues of Darth Vader: Dark Lord of the Sith by Charles Soule and Daniele Orlandini. The biggest question that arises about Vader's castle is why Mustafar? Why would the Lord of the Sith construct his castle on the planet where he lost his human side in the duel of the fates with Obi-Wan? Darth Sidious poses this same question to Vader in Darth Vader #21.
Palpatine previously sent Vader to Mustafar in order to bleed his Kyber Crystals and turn his lightsaber red. He performed the bleeding at an old sith cave on Mustafar where he felt a deeper connection to the force than he ever felt before. Vader reveals to Palpatine that through this cave the impossible can become reality. Vader's greatest wish is to bring Padmé back to life and he believes he can accomplish this on Mustafar. Palpatine gives Vader the planet of Mustafar and sends him on his way with one of the Empire's greatest architects, Padme's former ship and the mask of Darth Momin.
The Empire's architect fails in her initial attempt at Vader's castle. She is then killed by her assistant who is possessed by the mask of Darth Momin. Vader confronts Momin, kills the Imperial that is possessed, and in the process unveils a hologram of his future castle. Vader finds a suitable host for the mask and Momin explains his new design. His design would incorporate the Sith cave to channel the dark side of the force. He also explains to Vader that if Vader constructs his fortress above the cave, he will be able to meet Padmé again.
Lord Vader is able to use the full force of the Empire to help construct his fortress, but it would not be a smooth operation. In Darth Vader #23, also by Soule and Orlandini, the first attempt is finished. Vader tries to channel the dark side of the force and open the door but it fails and the entire building is destroyed. This happens 7 more times as Momin continually tries different designs. After the fifth attempt, Vader kills the body Momin was possessing and replaces it with a new host every time the castle fails. Each time a castle was destroyed, lava storms would erupt around the planet sending the Mustafarians into an uproar. Finally, the ninth attempt would work but the Mustafarians sought revenge against the Empire for the storms. Vader the left the Sith cave and castle to confront the homeworlders. Momin saw his opportunity to open the door himself and brought his original body back from the other side. Vader later showed the true power of his fortress during the battle with the Mustafarians. He retreated back to the cave when the tide turned towards the Mustafarians. Furthermore, Vader was able to channel the force through the tower, causing lava to rain down on the Mustafarians, killing them.
Vader would also have to defeat Darth Momin after the Mustafarians, but he'd be able to overcome him as well. The Lord of the Sith would eventually get to see his lost love, only for it to be a trick by the dark side of the force. He then destroys the portal to the other side and returns back to his fortress. While his main goal of bringing Padmé back to life failed, he was still left with a powerhouse of a fortress. The powers of the dark side that would channel through the castle would make it the perfect base for Vader to plan his next move against the Jedi.